Injection molding machines and extruders, which perform injection molding or extrusion of a solid feed (resin material) subsequent to melting and plastication, are widely known for many years. Such injection molding machines and extruders are provided with screws for melting and kneading a resin material, and rotation of the screws promotes melting and mixing of the melted and plasticated resin material.
As an illustrative example of techniques relating to such screws, there is the technique disclosed in JP kokoku 43-24493. According to this technique, as is illustrated in FIG. 7, a mixing head 107 is arranged on a free end of a screw 101, and a narrow clearance is formed between a cylinder 103 and the mixing head 107.
Further, as is shown in FIG. 8A, plural grooves 107a,107b are helically formed on an outer peripheral wall 107d of the mixing head 107, and are alternately arranged so that they do not communicate with each other. FIG. 9 is an A--A section of FIG. 8A and, as is depicted in FIG. 9, the grooves 107a and grooves 107b are alternately formed at equal intervals in an approximately central part of the mixing head 107.
A description will now be made about these grooves 107a,107b with reference to FIG. 8B and FIG. 8C. Incidentally, FIG. 8B and FIG. 8C are A.sub.1 --A.sub.1 section and A.sub.2 --A.sub.2 section of FIG. 8A, respectively.
As is shown in FIG. 8A, an end of each groove 107a extends to a tapered portion 107c on a side of a forward end of the mixing head 107 (on a left side as viewed in the drawing), and as is illustrated in FIG. 8A and FIG. 8B, its opposite end terminates in the outer peripheral wall 107d without reaching a tapered portion 107e on a side of a rearward end of the mixing head 107 (on a right side as viewed in the drawing).
In contrast with the grooves 107a, each groove 107b, as depicted in FIGS. 8A and 8C, terminates at an end thereof in the outer peripheral wall 107d without extending to the tapered portion 107c on the side of the forward end of the mixing head 107. Further, as is shown in FIG. 8A, this groove is formed so that at an opposite end thereof, it extends to the tapered portion 107e on the side of the rearward end of the mixing head 107.
A brief description will next be made about operation of the plasticating screw 101 having the mixing head 107 described above. When an unillustrated feed is supplied from a feed hopper 108 shown in FIG. 7, the cylinder 103 is heated to a predetermined temperature by a heater free of illustration. Rotation of the screw 101 by an unillustrated screw drive unit propels the resin feed from the feed hopper 108 toward a forward end of the cylinder 103. By heat from the heater via the cylinder 103 and shear heating of the feed in the grooves of the screw 101, the resin feed in the cylinder 103 undergoes gradual melting.
In some instances, kneading and dispersion of the resin feed would not be achieved fully if the screw 101 were solely relied upon. Owing to the provision of the above-mentioned mixing head 107, sufficient melting and kneading can be performed. Described specifically, the feed which has entered the grooves 107b from the tapered portion 107e on the side of the rearward end of the mixing head 107 moves into the grooves 107a under rotation of the mixing head 107, and is then propelled from the grooves 107a to the tapered portion 107c on the side of the forward end of the mixing head 107. As a result of the movement of the feed from the grooves 107b into the grooves 107a as described above, the feed is subjected to further shear force and dividing action so that good kneading and dispersion can be achieved. By the way, this mixing head 107 is generally called the "Maddock type" and besides the above-mentioned one, a variety of Maddock mixing heads are known.
In the case of the Maddock mixing head 107 disclosed in JP kokoku 43-24493, etc., the clearance between the mixing head 107 and the cylinder 103 is very small. In the case of a resin feed insufficiently melted by the screw 101 or a resin, such as PVC (polyvinyl chloride), melted considerably at the mixing head 107, the resin pressure may become off-balanced in the direction of a periphery of the mixing head 107, so that a metal-to-metal contact may take place between the mixing head 107 and the cylinder 103, resulting in so-called galling wear.
The mixing head 107 is arranged to improve the melting, kneading and dispersion of a resin feed. Melted conditions may however differ locally in the direction of the periphery of the mixing head 107. Resin pressure which is acting in an angular direction on the mixing head 107 may be off-balanced accordingly.
This results in the occurrence of force so that the mixing head 107 is strongly urged in a radial direction. Therefore an inner wall of the cylinder and the mixing head 107 undergoes a metal-to-metal contact, leading to problems that galling wear may occur on the mixing head 107 and the resin may be deteriorated due to localized over-heating.
To cope with these problems, a technique is disclosed for the prevention of such galling wear, for example, in JP kokoku 6-84035.
According to this technique, a flight 202 of a screw 201 shown in FIG. 10 is provided at a top portion thereof with a tapered land 204' as depicted in FIG. 11 or with a stepped land 204 as illustrated in FIG. 12. Owing to the provision of the land 204 or 204', lubricating pressure is forced to occur by a melted resin material between a cylinder 203 and the screw flight 202, thereby avoiding galling wear between the screw flight 202 and the cylinder 203. Incidentally, FIG. 13 diagrammatically illustrates the state of occurrence of such lubricating pressure.
Further, JP kokoku 4-77611 also discloses a technique which makes it possible to avoid the above-mentioned galling wear. According to this technique, as is illustrated in FIG. 14, a flight is formed broader in a barrier start section or end section of a so-called barrier flight screw or in both of the sections, and this broadened top portion is provided with a tapered (sloped) or stepped recess 301 the radius of which changes from a small radius to a large radius in the direction of rotation of the screw, whereby lubricating pressure is produced by a melted resin to avoid galling wear.
However, if a top portion of a screw flight is provided with such a land only on a main part of a screw like the techniques disclosed in JP kokoku 6-84035 and JP kokoku 4-77611 described above, a mixing head rotates under poorly-balanced load with a small distance left, thereby involving a problem in that galling wear at the mixing head cannot be avoided completely.
The present invention has been completed in view of these problem, and has as an object thereof the provision of a plasticating screw which can avoid a metal-to-metal contact between an inner wall of a cylinder and a mixing head even when resin pressure becomes off-balanced at the mixing head.